HE/VHE Gamma-Ray in Extragalactic-Astrophysics: Observations, Models, and Cosmic Implications

Dear Colleagues,

This Special Issue focuses on high-energy (HE, 0.1–100 GeV) and very-high-energy (VHE, >100 GeV) gamma-rays—a frontier in extragalactic astrophysics that unlocks insights into the universe’s most extreme phenomena and physical processes. It aims to integrate three core pillars of HE/VHE gamma-ray research, starting with observational advances: highlighting breakthroughs from global facilities, including space-borne Fermi-LAT (HE), ground-based Cherenkov telescopes (e.g., H.E.S.S., MAGIC, VERITAS for VHE), China’s LHAASO (the world’s most sensitive TeV-beyond emission detector), and CTAO (Cherenkov Telescope Array Observatory). LHAASO’s recent contributions—such as detecting 1.4 PeV gamma photons (the highest-energy ever observed) and constraining the Extragalactic Background Light (EBL) via GRB 221009A—are particularly emphasized, bridging HE and VHE studies. The issue also encompasses theoretical models, addressing how these gamma-rays are produced (e.g., leptonic/hadronic processes) and propagated through intergalactic space, while resolving discrepancies between observations and existing frameworks (e.g., EBL absorption/attenuation effects). Additionally, it explores cosmic implications, linking gamma-ray data to cosmic ray origins, intergalactic magnetic field properties, and the evolution of extragalactic structures (e.g., active galactic nuclei, gamma-ray bursts, star-forming galaxies) across cosmic time. By uniting observers and theorists, and incorporating cutting-edge HE/VHE results from facilities, this issue seeks to consolidate current knowledge, identify open questions, and chart future directions for HE/VHE gamma-ray studies in unraveling the universe’s most energetic mysteries.

Dr. Zhiyuan Pei
Dr. Hubing Xiao
Guest Editors

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